A combustion chamber assembly unit for a fuel-operated vehicle heater, in which a combustion chamber defined by a bottom area and by a circumferential wall passes over in the area of a flame diaphragm into a waste gas flow space enclosed by a flame tube, is known from DE 103 46 558 B3. The waste gas flow space is axially open at an axial end of the flame tube and the combustion waste gases being released in this area from the flame tube enter a waste gas backflow space formed between the flame tube and a housing enclosing the flame tube. The combustion waste gases transfer heat to a heat carrier medium flowing past the housing on the side thereof facing away from the flame tube, for example, to the air to be introduced into an interior of a vehicle, or to the cooling liquid circulating in a coolant circuit of an internal combustion engine.